This application is based on French Patent Application No. 01 00 077 filed Jan. 4, 2001, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. xc2xa7119.
1. Field of the Invention
The present invention relates to a vent valve which can be used as a safety valve for a storage cell, for example an aqueous electrolyte storage cell. The invention also relates to a storage cell equipped with the vent valve.
2. Description of the Prior Art
Aqueous electrolyte storage cells, for example nickel-cadmium storage cells, generally include a casing that is sealed to prevent electrolyte from leaking out and oxygen and pollutants from entering. The electrochemical system of this kind of cell can give off large quantities of gas in operation, or the cell may become overheated. This increases the pressure of the gases and/or air trapped in the casing of the cell. In particular, this occurs if the cell is subjected to intense, or even abusive, operating conditions, under which it delivers excessive electrical currents. This increase in pressure can deform the casing of the cell and cause the casing to leak or even explode. To prevent this, storage cells are generally equipped with safety valves that evacuate the gases to the outside if the pressure inside the casing exceeds a given threshold.
There is a first type of safety valve that includes an elastomer member that is ruptured by the pressure inside the storage cell if it exceeds a particular threshold. This type of vent valve has the drawback that it can be used only once: once broken, the membrane no longer seals the cell, even if operating conditions return to normal.
A second type of safety valve includes a seal blocking an orifice in the casing of the storage cell. The seal opens to free the orifice and allow the gas to escape from the interior of the casing if the pressure exceeds a given level, and then closes again when the pressure inside the casing falls below that given level. Operation of this device relies on retaining the seal, which takes the form of a gasket, against the perimeter of the orifice in the casing by means of a resilient member such as a spring. This solution has the drawback of necessitating a large space to house the gasket and the spring. It also has a high unit cost because of the number of components and the need to assemble them to the cell.
FIGS. 1, 2a and 2b show a further solution. The vent valve 1 is on the outside of a cover plate 2. The cover plate 2 seals the top part of the casing of the storage cell, not shown. The vent valve 1 includes a vent cap 3 on the outside of the cover plate 2. The area of the cover plate 2 including the vent cap 3 is flat; the portion of the cover plate 2 shown diagrammatically in FIG. 1 is limited to this area. The vent cap 3 takes the form of a section of a circular cylindrical tube 4 delimited in the lengthwise direction by two planes perpendicular to its axis and one end of which is closed by a bottom 5. The other end of the section is fixed to the cover plate 2. The bottom 5 is flat and parallel to the flat area of the cover plate 2 including the vent cap 3. An orifice 6 in the cover plate 2 establishes communication between the inside of the casing of the cell and the inside of the vent cap 3. The orifice 6 is substantially coaxial with the cylindrical tube section 4. Two vent holes 7a and 7b are provided in the cylindrical tube section 4 at a level adjacent the bottom 5. The vent holes 7a and 7b are diametrally opposed. A seal 8 is provided inside the vent cap 3, to be more precise between its bottom 5 and the cover plate 2. The seal 8 is shown in FIGS. 2a and 2b. The seal 8 has the following external shape. It includes a part 9 in the shape of a circular cylinder section delimited in the lengthwise direction by two planes perpendicular to its axis. On each of the disk-shaped opposite faces of the part 9 is a respective frustoconical part 10a, 10b coaxial with the part 9. The diameter of the frustoconical parts 10a, 10b decreases from a diameter equal to the diameter of the part 9. Each of the frustoconical parts 10a, 10b is delimited by a plane perpendicular to the axis common to the part 9 and the frustoconical parts 10a, 10b. As a result the seal 8 has two parallel plane disk-shaped faces 11a, 11b. The seal 8 is made from an elastomer. At rest, when it is not fitted, the height of the seal 8xe2x80x94i.e. the distance between the two faces 11a and 11bxe2x80x94is greater than the distance between the bottom 5 of the vent cap 3 and the cover plate 2. On the other hand, the diameter of the part 9 of the seal 8 is less than the inside diameter of the tube section 4 of the vent cap 3. The seal 8 is placed inside the vent cap 3 with the face 11a pressed against the bottom 5 of the vent cap 3 and the other face 11b pressed against the cover plate 2. As a result the seal 8 is compressed between its two faces 11a and 11b. This is how the vent valve 1 works. Under normal operating conditions of the storage cell, the face 11b of the seal 8 is pressed elastically against the cover plate 2, around the orifice 6, because of the compression of the seal 8 between the bottom 5 of the vent cap 3 and the cover plate 2. Consequently, the seal 8 seals the orifice 6 in the cover plate 2. If the pressure inside the casing of the storage cells exceeds a given threshold, it further compresses the seal 8 against the bottom 5 to the point of allowing the gases to find a path for themselves between the face 11b and the cover plate 2 and thereby reach the free area defined between the circumference of the seal 8 and the tube section 4, whence the gases escape freely through the vent holes 7a and 7b to the external environment of the storage cell. The path taken by the gases is indicated by the arrows G in FIG. 1. When the pressure inside the casing returns to a value below the threshold, the seal 8 is again sealed to the cover plate 2, around the orifice 6. Consequently, the orifice 6 is blocked again.
This solution has a number of drawbacks. Firstly, the seal 8 is fabricated by injection/compression of an elastomer, which implies a high fabrication cost. To increase productivity, either the number of fabrication molds or the number of imprints per mold must be increased. Apart from the cost of the tooling, the second of these approaches causes variations in the height of the seal, and consequently variations in the pressure at which the vent valve 1 opens.
Furthermore, precise calibration of the pressure at which the vent valve 1 opens also depends on correct centering of the seal 8 relative to the orifice 6. This centering is achieved by the tube section 4 of the vent cap 5. The orifice 6 is centered relative to the tube section 4, which centers the seal 8. The seal 8 is centered in the tube section 4 by the part 9 of the seal 8, which has a diameter close to the inside diameter of the tube section 4. Nevertheless, for the vent valve 1 to be able to operate, the diameter of the part 9 of the seal 8 must be less than the inside diameter of the tube section 4 in all situations, in particular if the diameter of the seal 8 increases because of the compression of the seal 8 between its two faces 11a, 11b, due to the manner in which it is mounted, but also because of the action of the gases via the orifice 6. Otherwise, the circumference of the part 9 would be pressed against the inside surface of the tube section 4. There would then be a seal between these two components, which would restrict or even render impossible evacuation of gases toward the vent holes 7a, 7b in the event of an increase in pressure in the casing of the storage cell. Consequently, the vent valve would no longer function correctly. This solution therefore has a supplementary disadvantage in that the diameter of the seal 8 depends on two contradictory considerations: on the one hand, the diameter of the seal 8 must be as close as possible to the inside diameter of the vent cap 5, to center it relative to the orifice 6, and, on the other hand, the diameter of the seal 8 must be sufficiently less than the inside diameter of the vent cap 5 to allow sufficient degassing in the event of an increase in pressure in the casing of the storage cell.
U.S. Pat. No. 3,994,749 proposes a polygonal seal die-cut from a sheet of elastomer. However, that seal has the drawback of giving rise to a reliability problem in the automated dispensing of components during assembly of the vent valve. There is the risk of the seal becoming wedged between the rails or in the vibrating bowls, unlike a circular seal, which circulates easily.
An object of the present invention is to propose a seal that does not have the drawbacks of the seals described in the prior art and which enables more reliable automated dispensing of components, in particular using existing industrial plant.
To this end, the present invention proposes a vent valve including an elastic seal compressed by a first wall against an orifice in a second wall and surrounded by a third wall, which seal has a section in a plane parallel to the first wall or to the second wall having three vertices disposed in a triangle, the three vertices of the section of the seal being connected in pairs by respective circular arcs.
Each of the circular arcs is preferably on the outside of an imaginary triangle defined by the three vertices. Also, the radius of curvature of each of the circular arcs can advantageously be less than or equal to twice the length of a side of the imaginary triangle formed by the three vertices. Furthermore, the radius of curvature of each of the circular arcs is preferably greater than or equal to Lxc3x97(3+3)/6 where L is the length of a side of the imaginary triangle formed by the three vertices. It is particularly advantageous if the radius of curvature of each of the circular arcs is equal to the length of one side of the imaginary triangle formed by the three vertices.
The third wall preferably has a circular section. The diameter of the circle circumscribed on the three vertices of the section of the seal, when measured free of all external forces, is preferably greater than or equal to 0.8 times the inside diameter of the third wall. The diameter of the circle circumscribed on the three vertices of the section of the seal, when measured free of all external forces, can with greater advantage be greater than or equal to 0.9 times the inside diameter of the third wall. It is especially preferable if the diameter of the circle circumscribed on the three vertices of the section of the seal, when measured free of all external forces, is equal to the inside diameter of the third wall.
The first wall is preferably parallel to the second wall. The respective faces of the seal coming into contact with the first wall and the second wall are preferably parallel. The flanks of the seal defining the section are preferably all perpendicular to a common plane. The flanks of the seal defining said section are preferably all perpendicular to two faces of the seal respectively in contact with the first wall and the second wall.
The vertices of the section of the seal are preferably disposed in an equilateral triangle.
The orifice in the second wall is preferably centered on the axis of the third wall. Furthermore, at least one vent opening can be formed in the third wall or in the first wall. The valve preferably includes two vent openings in the third wall or in the first wall and diametrally opposed relative to the axis of the third wall.
In a preferred embodiment the distance between the first wall and the second wall is greater than or equal to the diameter of the circle circumscribed on the three vertices of the section of the seal, as measured free of all external forces, preferably greater than or equal to twice the diameter of the circle circumscribed on the three vertices of the section of the seal, as measured free of all external forces, and advantageously greater than or equal to three times the diameter of the circle circumscribed on the three vertices of the section of the seal, as measured free of all external forces.
Furthermore the seal can include a plurality of stacked layers of materials.
In another aspect the invention provides a storage cell including a vent valve according to the invention. The second wall can be part of a cover plate closing the storage cell. Furthermore, the second wall or the third wall can constitute a terminal of the storage cell.